1. Field of the Invention
The present invention relates to a color copying apparatus, and more particularly to a color copying apparatus including an analog printer and a digital printer for preparing a reproduced image by determining exposure amount of a color original image.
2. Description of the Related Art
It is known from the experiential rule that when an original image recorded on a color negative film is copied onto a copying material such as a photographic printing paper, it generally suffices to set the ratio of the three colors of light, blue (B), green (G), and red (R) transmitted through the original image to a substantially apparatus, the exposure amount is determined on the basis of the following formula: EQU LogEj=Kj+Dj (1)
where LogE is the logarithm of the exposure amount; K is a constant; D is the density (e.g., LATD) of the original image photometrically measured by a photometric system; and j is any one of the colors, R, G and B. In addition, if the original image is printed by a photographic printing apparatus by using the exposure amount determined on the basis Formula (1) above, there are cases where, due to the characteristics of the photographic printing apparatus and the copying material, a print from an underexposed original image exhibits a higher overall density than a print from a properly exposed original image, while a print from an overexposed original image exhibits a lower density. For this reason, Dj in Formula (1) is corrected in correspondence with the density of the original image so as to determine the exposure amount.
However, with the exposure amount determined by using only the photometric data on the original image, the color and the density are not uniform among the prints, and prints with excellent finish cannot be obtained in some cases.
In the case or one film or a series or films in which a predetermined number of original images continue, the film characteristics are identical or similar, so that the characteristics of the original images in the frames to be printed (hereafter referred to as print frames) are also similar. A technique is known in which the exposure amount is determined by using average density values of original images and the densities of the respective original images concerning a series of film frames including print frames or one film frame including a print frame with respect to the image densities of the original images (refer to Japanese Patent Application Publication No. 112345/1976). Hence, it is possible to suppress variations in the color and density of one film or a series of films.
However, among the films, there are films whose film characteristics are in a standard state (hereafter referred to as normal films) as well as abnormal films whose film characteristics are in a different state (hereafter referred to as abnormal films). As examples of the abnormal films, it is possible to cite films whose characteristics have deteriorated over time, films including original images photographed by a photographing light source other than daylight, films developed in noticeably poor developing conditions among the developing conditions which vary among laboratories, and the like. With the above-described method, unless a discrimination is made between the abnormal films and the normal films in determining the exposure amount, the prints of either type of films cannot be finished with excellent quality.
In addition, Japanese Patent Application Publication No. 110829/1979 discloses a technique wherein, instead of using the average density values of original images of one film, a gray step image is burned in, and exposure is controlled on the basis of the density of the gray step image, which corresponds to the average density of print frames, and the densities of print frames. In this technique, prints can be made with high quality if the gray step image possesses average characteristics of a multiplicity of films with respect to one film or film type. However, the more the characteristics of the gray image deviate from the average characteristics, the more the print quality deteriorates. In addition, in a color copying apparatus, since the average characteristics cannot be distinguished, the prints which are obtained are not finished with excellent quality in some cases.
Further, there are cases where frames of special original images whose characteristics differ from those of other original images (hereafter referred to as the abnormal frames) are included in both the normal films and the abnormal films. If an attempt is made to obtain prints from original images on such abnormal frames, since the exposure amount is determined by characteristics similar to those of the other original images, prints with excellent finish cannot be obtained.
Generally, it is well known that if the exposure amount is determined by high color correction, an excellent print can be obtained from a frame which is considered to be an abnormal frame. However, in the event that an error has occurred in distinguishing the abnormal frame, color failure would occur and a print of an undesirable color would be formed. Accordingly, high accuracy in the distinction of the abnormal frames is required.
In order to detect the aforementioned abnormal frames, a technique has been proposed in which characteristic values (maximum densities of respective colors in the image plane, an area of a high-saturation color, etc.) for distinguishing abnormal original images are determined for one film including print frames, and an abnormal frame is detected by using an average value of these characteristic values (Japanese Patent Application Laid-Open No. 211136/1982). In the same way as the abnormal films, included among these abnormal frames are original images recorded on a film deteriorate over time, original images recorded on a film photographed by a photographing light source other than daylight, and original images recorded on a film which was developed under developing conditions which vary among laboratories. This technique requires much time and labor in the development of a determining method for detecting abnormal frames, so that this technique has not been efficient. In addition, this technique had a drawback in that an erroneous determination can be made in the case of a film in which a subject with a biased or predominant color such as the blue sky has been photographed.
In addition, Japanese Patent Application Laid-Open No. 319933/1992 discloses a technique of determining the exposure amount on the basis of average values of photomeric values of a plurality of frames of a film concerning timer-burnt-in prints. However, this technique is applicable to cases where standard subjects have been photographed in the plurality of frames, and it is difficult to accurately determine the exposure amount in the case of images containing subjects with a biased color which is liable to produce color failure or the like. Also, it is very difficult to properly determine an exposure amount with corrections made in differences in development and variations in the film characteristics.
As a technique for overcoming this problem, Japanese Patent Application Laid-Open No. 110829/1979 discloses a technique wherein a gray step image is burned into a film, and exposure is controlled on the basis of the density of the gray step image and the densities of print frames. In this technique, a gray step image is test-exposed on the color film before its development, and print frames are photometrically measured after development. At the same time, density characteristics concerning the three primaries of the gray step image are measured, an average density of density steps of the gray step image is determined, and exposure amount is then determined by using as a reference the density of the gray step which is closest to the average density of the gray color of the print frame. In addition, the exposure amount can be determined by multiplying the average density of the density step used as the reference and the average density of the print frame by weighting factors.
With this technique, f the relative spectral distributions of the illuminating light at the time of test exposure and the light from the subject (hereafter referred to as the photographing light) leading to the film after being transmitted through a camera lens accurately match, the density of the gray step image can be utilized in exposure control. However, if the relative spectral distributions differ, the print quality deteriorates. For example, in a case where the photographing light is sunlight, it is considerably difficult to allow the artificial light (illuminating light) at the time of test exposure to match the sunlight.
In addition, to burn in the gray step image serving as a reference, it is necessary to steadily obtain stable illuminating light regardless of changes over time and in the environment. The exposure apparatus for performing test exposure is required to be managed so that a fixed exposure amount can be obtained at different times in one day or over an extended period of time or even when a change has taken place in the ambient environment. Also, the exposure apparatus is required to be provided with a managing mechanism for such a purpose. In addition, as for such exposure apparatuses, it is necessary to eliminate machine differences among a plurality of exposure apparatuses. That is, the print quality may possibly decline substantially if, in a printer at the time of a reorder or make-over, the exposure amount is determined on the basis of the gray step image used in an exposure apparatus having a machine difference with the exposure apparatus used in the printing of the gray step image.
Furthermore, prints can be made with high quality if the gray step image possesses average characteristics of films. However, the more the characteristics of the gray image deviate from the average characteristics, the more the print quality deteriorates. For example, the tone of color becomes biased, i.e., a certain color undesirably tends to predominate, when the color temperatures of daylight do not match due to a difference of the season or the photographing district. In such a case, the print quality may possibly deteriorate, and it is therefore necessary to cause the color temperatures to match.
To overcome these problems, a measure can be provided by resetting the printing conditions on a trial-and-error basis each time the print is made, but the operation and processing become complex, so that this measure is not practical.